Muilenburg: Returning Max to service ‘will be an international decision’

Boeing CEO Dennis Muilenburg says the successful return to service of the company’s 737 Max series depends on international consensus among the many national aviation authorities (NAA) that will see the aircraft operating in their countries.

Not just the US FAA.

As a reminder, the 737 Max series fleet was grounded in March as a result of findings from the investigations into to the Lion Air and Ethiopian Airlines fatal crashes, respectively in October 2018 and March this year.

Speaking this week at Boeing’s Seattle Delivery Centre, Muilenburg declined to predict a return-to-service date, explaining: “Dates are uncertain because we are going for a global recertification.” That means unanimity – near or absolute – has to be achieved.

Boeing aircraft being prepared for delivery at Boeing Field, Seattle

He emphasised the point: “If we do not coordinate this [return to service] we may see some disaggregation, and I don’t think that’s a future any of us wants to see.”

Muilenburg is confident the combined hardware and software changes Boeing has developed for the Max will satisfy the FAA and the multinational Joint Operations Evaluation Board (JOEB).

The primary causal factor of the Lion Air crash was erroneous triggering of its manoeuvring characteristics augmentation system (MCAS) by a faulty angle of attack (AoA) sensor, according to the Indonesian final accident report. It is at the MCAS that Boeing’s efforts have been directed.

More on MCAS later.

Boeing test pilot and VP Operations Craig Bomben, who flew the 737 Max first flight and has coordinated development activity on the type since the accidents, described the essential difference between the original MCAS and Boeing’s proposed replacement: “We’ve moved from a very simple system to an intelligent system.”

In both the accidents MCAS – triggered by a faulty or damaged AoA sensor which wrongly indicated a high AoA – reacted by providing nose-down stabiliser rotation that took the pilots by surprise. They did not understand the reason it kicked in. Their efforts to reverse the strong nose-down pitch did not succeed, and because both these events occurred just after take-off, the loss of height quickly resulted in impact with the surface.

Bomben said the new “intelligent” system has two AoA sensors instead of one, and if their readings differ by 5.5deg or more, MCAS is not triggered at all.

But if it is correctly triggered, the system now “operates only once per AoA event”, according to Bomben, and when it does trigger stabiliser movement, it memorises how much displacement has taken place, so if it were triggered again it would take account of existing stabiliser displacement and will not apply more than a safe cumulative limit.

But why is MCAS – which is unique to the Max – required at all? Boeing insists it was not fitted as an anti-stall system, because the aircraft already has stall warnings and stick-shakers.

The purpose of fitting MCAS, Bomben explained, was to compensate for a slight change in the low-airspeed aerodynamics of the 737 Max compared with the NG.

MCAS was only designed to trigger in an unlikely (but obviously possible) combination of circumstances that can cause the aircraft’s centre of lift to move slightly further forward, altering the weight-balance equation. It only happens when the Max is at low airspeed with the flaps up, and is being flown manually.

At low airspeed (200kt or thereabouts) – and flapless – the aircraft would be at a high angle of attack and close to the stall. FAA regulations require that one of the cues to the pilot of the approaching stall is that there should be a linear increase in the required column force versus displacement response.

In the Max, however, at a certain point in this sequence the centre of lift shifts forward a little, providing a slight nose-up pitch force, therefore the stick force does not continue to increase, so MCAS is designed to kick in with some nose-down trim to restore the linear increase.

If MCAS doesn’t kick in, the aircraft is still easily controlled without it, but the required progressive stick-force cueing is lost.

In technical and regulatory terms, MCAS seems to be a lot of fuss for very little purpose, but the painful fact is that the original MCAS played its part in bringing down two aeroplanes and killing 346 people.

Muilenburg’s confidence in the fix is, so far, based on more than 100,000 hours of development work on the new solution, plus 1,850 flight hours using the new software, 1,200 hours of refining the results in the simulator, and 240 hours of regulatory scrutiny in the simulator.

Meanwhile, if Muilenburg cannot predict when the world will approve the 737 Max’s return to the air, what is happening to its production at present? The aircraft had won more than 5,000 orders, and fewer than 400 have been delivered.

The Max series, despite the grounding, continues to roll off the production line at Boeing’s Renton plant near Seattle, at a rate of 42 per month. The factory is capable of turning out 57 a month, but Boeing is keeping the rate lower for now. Despite this, Renton has seen no staff layoffs, says Boeing.

The completed aircraft, however, go into storage at Moses Lake or San Antonio desert sites, because the manufacturer’s own sites at Renton, Everett and Boeing Field are full.

Muilenburg said every 737 Max grounded or in store awaiting modification will have an individual entry into service programme, and that in the meantime the engines, systems and cabin of all the aircraft are regularly being run and maintained.

But will they still have that “new plane smell” when the airlines take delivery?

A Max in production at the Renton plant, its unique split winglet close to the camera


17 thoughts on “Muilenburg: Returning Max to service ‘will be an international decision’

  1. “But will they still have that “new plane smell” when the airlines take delivery?”

    Would that be the usual ‘wet dog’ or ‘vomit’ smell which is normally associated with rare contaminated bleed air with toxic oil (but in harmless very low concentrations) design flaw since the 1960’s?

    Obviously, we need not one but two 737 Max type fatal accidents to change completely to ‘bleed free’ architecture, as the clever Chinese have logically been doing for over 10 years…



    • True, John, but that failing is an affliction also of all the other modern airliners, with the exception of the Boeing 787, which was designed with an alternative system for pressurisation and air conditioning.


      • In 1999 the word Aerotoxic was first created but at exactly the same time that Dr Jean Christophe Balouet of France urged Boeing to change their architecture to non bleed air and they did. Why not just tell it straight?


  2. John, you are welcome to file more on aerotoxic syndrome if you wish, but I will not comment further on the subject in this space because this item is about other important safety matters. If you have anything to say about the problems revealed by the fatal Lion Air and Ethiopian Airlines accidents, please contribute.


  3. What the accidents and the NTSC report revealed is that far-eastern pilots lack training (You can see the pilot ‘fighting’ the MCAS system on the FDR plots at least 10 times. How long before you throw the Stab Trim Cutout switches?) and the NTSC investigators need to “show their work” next time; that report is 90% extraneous minutia, and unconnected facts (why is an 1-year old plane fixed with refurb components? Who approved that? Was _any_ Boeing SR contacted about this? Who in their right mind flies a plane with the stickshaker active for a full leg? Why was this not written up? The plane blew all 3 generator breakers while parked, the tech the next day pushed them back and released the plane.) I was cursing louder and louder as I read the report.


    • Circuit, you are right that the Indonesian report catalogues endless errors and failings in the crew, the airline’s maintenance procedures, and aircraft operations control, demonstrating a very poor safety culture at Lion Air. One of their crews saved the accident aircraft from the effects of wrongly-triggered MCAS, but demonstrated a chilling lack of airmanship; the same aircraft’s next crew failed to save it when MCAS again was triggered erroneously.

      But behind it all lies the fact that Boeing does not design and build its aircraft purely for American – or Western – pilots, and neither does Airbus or Embraer etc. It’s simply better not to design in potential traps, even if well-prepared pilots will normally cope with them.

      Liked by 2 people

      • There is no question that Boeing erred badly with not only the fundamental design of the MCAS system but also the fact that they did not provide training materials (at least initially) on how the system worked.

        That being said the focus on the airplane means that the central cause is going unaddressed. Poor Airmanship. A properly trained crew applying proper technique should have been able to deal with an MCAS failure. The MAX is the first airliner to ever be grounded for a technical fault that proper crew technique could safely manage. Unless and until world aviation authorities are willing to focus on making sure that pilots are capable of doing more than just managing an aircraft on autopilot we are going to keep having accidents that should not have been.

        Mr. Learmount a question if I may? In the US we are now flying simulators that have been modified to reflect proper post stall aerodynamics and we are being trained how to recover the aircraft from these flight regimes. This is true even for the A320 fleet that I fly where recent recurrent included recoveries where abnormal attitude law was active and the airplane was inverted and very nose low. Have European or any other regulatory agencies that you know of also began to mandate this kind of training?

        It’s been an interesting few years as annual training has been less and less focused on the traditional elements and more and more focused on flying the aircraft without automation and being able to safely recover from even the most extreme upsets.


      • 121Pilot, I know the aviation authorities have been pressing the simulator manufacturers to replicate as accurately as possible aircraft behaviour on the edges of the flight envelope – particularly right into the stall – but I don’t think they have mandated it yet. I will have to check that. One of the main motivations for pushing these capabilities, as you have implied from your recurrent training experience, is to enable upset recovery training in simulators to be a little closer to reality that it has been. Your airline obviously takes advantage of this capability, but unless the training exercises are mandated not all airlines will.


  4. […] Muilenburg: Returning Max to service ‘will be an international decision’ – Learmount.comBoeing held a gathering in Seattle last week for influencers, analysts, and really anyone but media. I was included in the group, but I was at a conference I couldn’t miss. This write-up seems to be the best one I’ve seen yet. It just points to a slow, slow, slow re-entry into service for the MAX. And on that note… […]


  5. David,

    My understanding is that at least in the US the reason these sim mods and training are happening is because the FAA has mandated it. That’s what I’ve been told by senior folks in the schoolhouse on more than one occasion. Its disappointing that other regulatory agencies are still lagging on this subject.


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